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Using a time invariant statistical process variable of a semiconductor chip as the chip identifierRelated Patent Categories: Semiconductor Device Manufacturing: Process, With Measuring Or Testing, Electrical Characteristic SensedUsing a time invariant statistical process variable of a semiconductor chip as the chip identifier description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060063286, Using a time invariant statistical process variable of a semiconductor chip as the chip identifier. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The invention relates to a semiconductor chip identifier or a method for providing identification for a semiconductor chip and, in particular, to a method of using an electrical parameter that is or is based on a time-invariant statistical process variable of the semiconductor chip as the chip identifier. DESCRIPTION OF THE RELATED ART [0002] In the manufacturing of semiconductor integrated circuits (also referred to as semiconductor chips) and in the incorporation of semiconductor chips into electronic systems, it is often necessary to provide identification for each chip. The identifier (ID) may be unique for each chip or for a group of chips. The identifier may be used to associate each chip with a set of repair data for that chip. The repair data can be used to enable repairing of a specific defect in the chip. The ability to associate repair data with an individual semiconductor chip is particularly useful for memory bit repair/replacement in a memory device or defective pixel repair in a CMOS or CCD image sensor. Alternately, chip identification capability can be used to enable (or disable as appropriate) certain features in the software associated with the electronics system in which the semiconductor chip is installed. For example, a microprocessor chip could be identified as licensed to run a particular operating system or application program and other chips of the same type, not identified as such, would be considered as not capable of supporting that functionality. [0003] Methods for providing identification for semiconductor chips are known and generally require some form of physical alteration of the chips to incorporate the identification number. The prevailing method for identifying a specific chip is accomplished by blowing fuses formed on the chip. The fuses can be blown either with a laser or electrically. Using fuses for identification requires either more complicated processing steps, such as additional fuse layers, or additional programming steps to blow the fuses. Adding and programming fuses add to the cost of manufacturing the semiconductor chip. A more cost effective method for providing chip identification is desired. SUMMARY OF THE INVENTION [0004] According to one embodiment of the present invention, a method for providing an identifier for a semiconductor chip after the manufacture of the semiconductor chip using a fabrication process includes selecting one or more circuit elements formed on the semiconductor chip where each of the circuit elements having an electrical parameter that has a time-invariant statistical process variation, measuring data values of the electrical parameter of the one or more circuit elements, processing the data values, and deriving the identifier for the semiconductor chip using the processed data values. The identifier identifies the semiconductor chip from other semiconductor chips manufactured using the fabrication process. [0005] In one embodiment, selecting one or more circuit elements formed on the semiconductor chip includes selecting one or more circuit elements from the group of bipolar transistors, MOS transistors, light detecting pixel elements, memory cells, resistors, capacitors and inductors. [0006] The method of the present invention can be advantageously applied to provide unique identification for image sensor chips. According to one embodiment of the present invention, a method for providing an identifier for an image sensor semiconductor chip including an array of pixels after the manufacture of the image sensor semiconductor chip using a fabrication process includes measuring dark current values of each pixel in the array of pixels, selecting N pixels from the array of pixels having the N highest dark current values, obtaining the locations of the N pixels in the array of pixels, serializing the location values of the N pixels into a bit string, and providing the bit string as the identifier for the image sensor semiconductor chip. [0007] In an alternate embodiment, the N highest dark current values are digitized into k-bit digital values and the k-bit digital values are serialized to form the identifier for the image sensor semiconductor-chip. [0008] The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0009] FIG. 1 is a flow chart illustrating the chip identification method according to one embodiment of the present invention for assigning an identification number to a semiconductor chip. [0010] FIG. 2 is a flow chart illustrating a look-up routine for chip identification. [0011] FIG. 3 illustrates a chip identification method for an image sensor chip according to one embodiment of the present invention. [0012] FIG. 4 illustrates the steps that can be taken at the camera assembly stage for retrieving the stored calibration data associated with an image sensor chip. [0013] FIG. 5 illustrates pixel patterns for two image sensors for illustrating the random nature of the dark current values. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0014] In accordance with the principles of the present invention, a method for providing identification for a semiconductor chip uses an inherent electrical parameter of the semiconductor chip having a time-invariant statistical process variation as the chip identifier. The electrical parameter, which can be a time-invariant statistical process variable or based on a time-invariant statistical process variable, is measured after the chip is manufactured and the measured data is processed to be used as a unique identification number, such as a serial number, for each semiconductor chip. In some embodiments, the threshold voltage of a transistor or the dark current of a light detecting pixel element is the electrical parameter being measured and processed to derive an identifier for the semiconductor chip. In this manner, each semiconductor chip can be uniquely identified without requiring additional fabrication process steps and without requiring physical alteration to the chip to label the chip. [0015] In the present description, an electrical parameter having a time-invariant statistical process variation or a time-invariant statistical process variable refers to a random variable inherent in the semiconductor chip that has a statistical distribution when the semiconductor chip is manufactured in a given fabrication process. An electrical parameter has a statistical process variation when, despite methods employed to curtail process variations, the fabrication process used to manufacture the chip results in the electrical parameter having values that are statistically distributed over a certain range. The chip identification method of the present invention exploits the random nature of these statistical process variables for use as identification for semiconductor chips. [0016] The chip identification method of the present invention uses an electrical parameter associated with one or more circuit elements that are built on the semiconductor chip. A variety of circuit elements can be used to practice the method of the present invention and the term "circuit element" as used in the present description applies to electrical devices (such as resistors or transistors) and to circuit units built using one or more electrical devices (such as a memory cell or a light detecting pixel element or an amplifier). In practice, any circuit element on the semiconductor chip that has an inherent electrical parameter having a statistical process variation can be used as a source of identification for the semiconductor chip. In some embodiments, the circuit elements used are bipolar transistors, metal-oxide-silicon field effect transistors (MOS transistors), light detecting pixel elements ("pixels"), resistors, capacitors and inductors. In other embodiments, the circuit element can be a memory cell (such as an SRAM cell) where the intrinsic logical value of the memory cell upon power up is the time-invariant statistical process variable used to identify the semiconductor chip in which the memory cell is formed. The circuit element can also be an oscillator where the center frequency of the oscillator, being a function of the inductance and the capacitance of the oscillator circuit, is the time-invariant statistical process variable used to identify the semiconductor chip in which the oscillator is formed. Furthermore, the circuit element can be an amplifier where the gain of the amplifier is the time-invariant statistical process variable used to identify the semiconductor chip in which the amplifier is formed. [0017] The circuit element used in the chip identification method of the present invention can be a dedicated circuit element or a non-dedicated circuit element. In one embodiment, the method of the present invention is practiced by providing a dedicated set of circuit elements that have no function other than to provide an identifier for the semiconductor chip. The provision of dedicated circuit elements in the semiconductor chip for the chip identification method of the present invention does not increase the processing complexity of the semiconductor chip as the circuit elements can be manufactured using the same fabrication process steps used to manufacture the semiconductor chip. In an alternate embodiment, the method of the present invention is practiced by using circuit elements that are built in the semiconductor chip for performing chip-specific functions. In this case, no additional circuit structures need to be added to the semiconductor chip for implementing the chip identification method of the present invention. [0018] Depending on the circuit elements selected, the electrical parameter having time-invariant statistical process variation (the time-invariant statistical process variable) that is measured can include a variety of parametric values that are indicative of the electrical characteristics of the circuit elements. In some cases, the electrical parameter can be the time-invariant statistical process variable itself, such as a voltage or a current value. In other cases, the electrical parameter can be a parameter based on or derived from one or more time-invariant statistical process variables, such as a natural logical state of a circuit element or the gain of a circuit element. [0019] Thus, in some embodiments, the time-invariant statistical process variables used for the chip identifier include the threshold voltage of a MOS, the base-to-emitter voltage (V.sub.BE) or the gain .beta. of a bipolar transistor, the drain current of a MOS transistor, and the collector current of a bipolar transistor. Alternately, the resistance value of a resistor at a given excitation, the capacitance value of a capacitor at a given excitation and the inductance value of an inductor at a given excitation can also be used. In another embodiment, the gain of an amplifier is used as the electrical parameter for identification where the gain is a function of one or more time-invariant statistical process variables of the amplifier circuit. Continue reading about Using a time invariant statistical process variable of a semiconductor chip as the chip identifier... 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